Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Ragwort Biocontrol Pays Off

An increasingly unfamiliar sight in New Zealand, ragwort in 1981 before the release of the flea beetle.

An increasingly unfamiliar sight in New Zealand, ragwort in 1981 before the release of the flea beetle.

Ragwort (Jacobaea vulgaris) spread to reach its full potential range in New Zealand back in the 1920s, infesting vast areas of pastoral land. Especially toxic to horses and cattle, this poisonous weed was a particular problem for dairy farms around New Zealand, and was one of the first biocontrol projects in New Zealand.

During the 1920s, government agencies took the advice of a Professor of Botany in the UK and chose the cinnabar moth (Tyria jacobaeae) and a seedfly (Botanophila jacobaeae) as biocontrol agents to try. The ragwort flea beetle (Longitarsus jacobaeae) was shortlisted in the 1930s, but then dismissed as a potential agent because it was thought to have little impact on ragwort, based on observations that the adults only cause minor damage to leaves, and overlooking the fact that the larvae can severely damage the roots. By 1939, both the cinnabar moth and the seedfly were well established, but failed to make any noticeable progress on the ragwort problem. It wasn’t until 1980s that the idea of using the ragwort flea beetle was revisited based on encouraging reports from the USA and Australia where the beetle had been introduced to control ragwort. The ragwort flea beetle was released in New Zealand in 1983 and by the early 1990s was beginning to make inroads into the ragwort problem, which is now a rare sight in many previously infested parts of the country. “Although the beetle had a big impact on the ragwort, unfortunately, little quantitative data was collected or published so until now, we have only been able to speculate on the financial benefits to New Zealand farmers,” explained Simon Fowler.

In 2005, two additional insect biocontrol agents, the ragwort plume moth (Platyptilia isodactyla) and the ragwort stem borer (Cochylis atricapitana), were released in New Zealand to complement the ragwort flea beetle at wetter sites where it is less effective. Both of these moths have been used successfully in Tasmania to improve the levels of control achieved by the flea beetle, although in New Zealand only the plume moth has established. As part of the application to release these two agents, a survey was conducted on 32 randomly selected West Coast dairy farms (where the ragwort flea beetle had not provided sufficient control) to determine the cost of ragwort control.

In the past few months, Simon has used this data to complete a national benefit-cost analysis that predicts what the cost of ragwort control would be across New Zealand in the absence of biocontrol by the flea beetle. “The ragwort flea beetle failing to suppress ragwort on the West Coast has given us a chance to extrapolate what the cost might be nationally if there wasn’t successful biocontrol of ragwort in place elsewhere,” explained Simon. This is only the second time that a post-release economic analysis of a New Zealand weed biocontrol programme has been done, the first being the analysis of St John’s wort (Hypericum perforatum) (see Issue 61).

The costs of developing the biocontrol programme were calculated from excellent historical reports on the research carried out, and then inflation-adjusted back to the year they were incurred. Ragwort control costs (chemical and manual), were extrapolated from the 2005 survey of dairy farms on the West Coast, taking into account the proportion of farms nationally that had benefited from the flea beetle (based on a aggregation of all the quantitative data we could compile). Further adjustments were made for inflation and the national size of the dairy herd from 1926 to the present day.

The economic analysis was undertaken on the assumption that the ragwort control costs, reported by the farmers in the 2005 survey, were accurate and indicative of costs that would have been incurred elsewhere in New Zealand in the absence of the flea beetle.

“We also assumed that the decline in ragwort elsewhere in New Zealand could be attributed to the presence of the ragwort flea beetle, and that where ragwort was suppressed it would be replaced with pasture and not some other invasive weed,” said Simon.

The results of the analysis took everyone by surprise. The savings in ragwort control costs on dairy farms in New Zealand as a result of biocontrol by the flea beetle was predicted to be $44 million for 2015 alone. These annual savings are ongoing and sustainable, with no further investment needed. A net present value analysis of the annual benefits and costs from 1926 onwards gave a benefit-cost ratio of 14:1, i.e. for every dollar invested in ragwort biocontrol New Zealand has gained $14 in reduced ragwort control costs. Who wouldn’t invest $1 to make $14! “Nevertheless, dairy farms nationally are still incurring costs of around $20 m each year, mostly in the wetter regions, to keep ragwort in check, but the establishment of the ragwort plume moth should help reduce this considerably,” Simon added.

“Despite some ongoing costs in high rainfall areas, the biocontrol of ragwort has had very large benefits to New Zealand dairy farms in terms of reduced control costs. Especially considering that these costs don’t include figures for loss of production or the costs to farmers when stock are poisoned,” Simon said. “Had we included the benefits from ragwort biocontrol to other sectors of the farming community, such as deer or sheep and beef farming, the savings would be even greater,” he added. Unfortunately, the decision to overlook the ragwort flea beetle as a biocontrol agent in the 1930s was very costly. Had it been introduced early in the programme, New Zealand would have saved a staggering $8.6 billion in today’s terms (calculated as net present value).

The economic analysis has underlined the importance of selecting appropriate agents at the start of biocontrol programmes rather than relying on anecdotal, non-quantified evidence from researchers and professors! “Refining agent selection is a key area of our research,” said Simon. “We want to avoid spending valuable stakeholder resources on agents that have a low chance of success and we have made considerable progress on understanding why around three of every four agents released and established in the past failed to have any significant impacts on the target weeds,” concluded Simon.

This project is funded by the Ministry of Business, Innovation and Employment as part of Landcare Research’s Beating Weeds programme. The project by the West Coast Ragwort Control Trust to bring in additional ragwort agents was supported mainly by the MAF Sustainable Farming Fund with contributions from a range of other organisations.